Move webrtc::AudioProcessing include to api/ folder

Bug: webrtc:15874
Change-Id: Ie8a6e031c0f0505cfe238f7d252c47e9c34408d4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/347983
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42128}
diff --git a/modules/audio_processing/gain_control_impl.cc b/modules/audio_processing/gain_control_impl.cc
index 5f2b487..2a7f527 100644
--- a/modules/audio_processing/gain_control_impl.cc
+++ b/modules/audio_processing/gain_control_impl.cc
@@ -13,9 +13,9 @@
 #include <cstdint>
 
 #include "absl/types/optional.h"
+#include "api/audio/audio_processing.h"
 #include "modules/audio_processing/agc/legacy/gain_control.h"
 #include "modules/audio_processing/audio_buffer.h"
-#include "modules/audio_processing/include/audio_processing.h"
 #include "modules/audio_processing/logging/apm_data_dumper.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"